"The immediate concern is North Korea. Another possible problematic area would be the Taiwan Strait," a Japanese official told Reuters, referring to a possible clash between China and Taiwan, viewed by Beijing as a renegade province.
"These are two tangible examples of possible threats to the peace and security of Northeast Asia," he said.
"As far as these threats are concerned, the United States and Japan will certainly express their view."
The Washington Post reported on Friday that the two allies would, for the first time in a joint agreement, identify security in the Taiwan Strait as a "common strategic objective".
Such phrasing would be likely to anger China just as Tokyo, Washington and Seoul are hoping Beijing can prod North Korea to return to six-party talks on its nuclear arms programmes.
North Korea stunned the world last week when it announced it had made nuclear weapons and was withdrawing from the talks.
"The United States and Japan will exhibit their very deep concern over North Korea's latest statement ... but at the same time, there will be very strong wording of our commitment to the six-party talks," the Japanese official said.
"We also expect further efforts by China to bring North Korea into a resumption of the six-party talks," he added.
Pyongyang's biggest backer, China, is trying to convince North Korea to return to the talks, which also include South Korea, the United States, Japan and Russia.
"We are concerned about the Chinese build-up because we don't know what direction it will take. But we can't afford a hostile China and would like China to be on our side to build a better international environment," said a Japanese military source.
"How to address these two issues is very sensitive."
Saturday's US-Japan talks are a necessary prelude to decisions on how to realign the nearly 50,000 US forces in Japan, part of a global transformation of America's military to cope with new threats such as terrorism.
Japan is leaning towards a bigger role as a strategic hub from which US forces can respond to threats in an "arc of instability" from the Middle East to the Koreas, a shift seen by many experts as an expansion of the alliance.